Exciplex Systems Research Articles

Dendritic Oligomers‐Based Exciplex Emitters Realizing Solution‐Processed Organic Light‐Emitting Diodes with Nearly 30% External Quantum Efficiency

AbstractDue to the feature of the strong intermolecular interactions causing aggregation‐caused excitons quenching (ACQ) of solution process, realizing high‐performance solution‐processed organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge nowadays. To address this issue, herein, a novel strategy is proposed to construct solution‐processable ternary exciplex emitters by utilizing the dendritic oligomer as the substrate, simultaneously realizing excellent solution‐process adaptability, good dispersibility to restrain harmful ACQ, and improved exciton utilization with multiple reverse intersystem crossing channels. Accordingly, dendritic oligomer‐based exciplex systems are constructed by combining small‐molecule donor and acceptor components with the multi‐carbazole‐substituted tetraphenylsilane‐core analog as substrate. And their film morphologies, photoluminescence quantum yields (ΦPLs), and non‐radiative decay rates of triplet excitons (knrTs) are significantly improved. Among them, DMAC‐DPS:PO‐T2T:SimCP3 exhibits the highest ΦPL of 97.5% and the lowest knrT of 1.0 × 103 s−1, and realizes the best maximum external quantum efficiency of 29.7% in solution‐processed OLED, which is comparable with the highest electroluminescence efficiencies of all solution‐processable emitters. Therefore, this work will provide a prospective pathway to develop efficient solution‐processable exciplex emitters and promote the progress of solution‐processed OLEDs.

Effective exciplex system with high emission efficiency via intramolecular hydrogen bonding for efficient solution processable OLEDs

Effective exciplex system with high emission efficiency via intramolecular hydrogen bonding for efficient solution processable OLEDs

New Concept for HLCT Emitter: Acceptor Molecule in Exciplex System for Highly Efficient and Extremely Low-Efficiency Roll-Off Solution-Processed OLED

New Concept for HLCT Emitter: Acceptor Molecule in Exciplex System for Highly Efficient and Extremely Low-Efficiency Roll-Off Solution-Processed OLED

Unexplored Exciplex Formation by Dual Excitation of Donor and Acceptor Layers Optically and Electrically

AbstractElectron donor and acceptor interactions are the fundamentals for the emergence of optoelectronic functions in most organic devices. In particular, exciplexes, which hold the possibility to utilize dark triplets, are widely investigated in organic light‐emitting diodes. However, the relatively low radiative decay rate with a low photoluminescence quantum yield poses further advanced challenges. Traditionally, it is well recognized that exciplexes are formed through a charge transfer process, i.e., direct electron transfer, by photoexciting either a donor or an acceptor component. In this study, the focus is on a long‐range exciplex system where tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA) and 2,4,6‐tris[3‐(diphenylphosphinyl)phenyl]−1,3,5‐triazine (PO‐T2T) serve as the donor and acceptor, respectively. Moreover, 1,2‐bis(triphenylsilyl) benzene (UGH2) and bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO), having a wide energy gap and a high triplet energy level, are employed as the double insulating layer to suppressing direct electron transfer between the donor and acceptor. The indirect through‐space exciplex formation is demonstrated by the simultaneous excitation of both donor and acceptor layers optically and electrically, providing a new route for exciplex formation.

Stepwise Charge/Energy Transfer in MR-TADF Molecule-Doped Exciplex for Ultralong Persistent Luminescence Activated with Visible Light.

Organic long-persistent luminescence (OLPL), which relies on energy storage for delayed light emission by the charge separation state, has attracted intense attention in various optical applications. However, charge separation (CS) is efficient only under ultraviolet excitation in most OLPL systems because it requires a driving force from the large energy difference between the local excited (LE) and charge transfer (CT) states. In this study, a multiresonance thermally activated delayed fluorescence (MR-TADF) molecule is incorporated into an exciplex system to achieve efficient OLPL in a composite material activated by visible light via a stepwise charge/energy transfer process. The enhanced absorption of the composite material facilitated a tenfold increase in the duration of the OLPL, which can last for several hours under visible light excitation. The excited state of the MR-TADF molecule tends to charge transfer to the acceptor, followed by energy transfer to the exciplex, which benefits from the small difference between the LE and CT states owing to the inherent CS characteristics of the opposing resonance effect. Afterglow displays of these composite materials are fabricated to demonstrate their considerable potential in encryption patterns and emergency lights, which take advantage of their excellent processability, visible light activation, and tunable luminescence properties.

Non-conventional exciplex system by leveraging zero-radius of intramolecular energy transfer mechanism: Seamless integration of p-host and fluorescent dopant for highly efficient OLEDs

A multi-component emissive layer (EML) composed of a mixture of p- and n-type hosts and a dopant is a widely used structure in organic light-emitting diodes (OLEDs) due to its high efficiency and operational stability. However, multi-component EML devices impose limitations such as difficulty in accurately controlling the mixing ratio and profiles in the manufacturing process, high costs, and complexity in the photophysical properties of the components.In this paper, a p-type host fused with a blue fluorescent dopant, 5,9-di([1,1′-biphenyl]-2-yl)-2′-(9H-carbazole-9-yl)3,7-dimethyl-5,5a,5a1,9,9a,16a-hexahydrospiro[5,9-diaza-16b-boraindeno[2,1-b]naphtho[1,2,3-fg]anthracene-11,9′-fluorene] (DABNA-Spiro-Cz), was designed and successfully synthesized to construct a novel EML structure. Completely fused within DABNA-Spiro-Cz, the p-type host and blue fluorescent dopant can independently perform their respective roles. An exciplex is formed between n-type host and p-type host unit in the DABNA-Spiro-Cz molecule and its energy is effectively transferred to the blue fluorescent dopant unit in DABNA-Spiro-Cz. In other words, a degree of freedom is earned from a conventional 3-component exciplex-dopant system. A high external quantum efficiency of 14.9 % was achieved thanks to “zero-radius of intramolecular energy transfer in exciplex (ZETPLEX)” system, which is completely non-conventional and novel. We believe the ZETPLEX mechanism opens a new era of designing emissive materials and paves a new way of constructing a novel EML and device architecture for highly efficient OLEDs.

Theoretical insights into long-range coupling of electron-hole pairs in TCTA–PO-T2T exciplex

This work reports theoretical investigations concerning long-range coupling of electron-hole pairs in spatially separated exciplex (SSE) systems via Density Functional Theory and Time-Dependent Density Functional Theory. Based on TCTA−PO-T2T parent exciplex, where 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) serves as donor (D) and 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) as acceptor (A), SSEs are constructed by intentionally tuning the D−A distance. The calculation results demonstrate that all SSEs, even if the D−A distance is over 14 Å, exhibit the clear and strong charge transfer character in terms of DCT, qCT, and t indexes. As the D−A distance increases, the energy gap of SSEs is increased, resulting in the blueshift of emission spectra. Calculation and experiment results show a good consistency, indicating that our model could well describe SSEs. Meanwhile, the reduced energy gap between CT and 3LE and more degenerate states could boost reverse intersystem crossing via vibronic coupling and hyperfine coupling, eventually improving the electroluminescent performance of SSEs. Our study suggests that manipulation of relative energy alignment via controlling D−A distance not only promotes the properties of exciplexes, but also offers guidance for designing thermally activated delayed fluorescence emitters through-space charge transfer.

Achieving Seconds‐to‐Hours Duration‐Tunable Organic Long Persistent Luminescence from Carbon Dots‐Based Exciplex Systems by Energy Gaps Regulation†

Comprehensive SummaryDuration‐tunable afterglow materials have garnered considerable attention in various applications. Herein, carbon dots (CDs)‐based long persistent luminescence (LPL) composites with a tunable duration in an ultrawide range of seconds‐to‐hours levels were designed and prepared for the first time. In contrast to the established CD‐based afterglow materials, we reported that CD‐based composites exhibit LPL in the form of exciplexes and long‐lived charge‐separated states, enabling the LPL to be prolonged from several seconds to over one hour, exceeding the typical regulation range (limited to 1 min). Further studies revealed that the relationship between the excited and charge‐transfer states of CDs plays a pivotal role in activating the LPL and regulating its duration. Furthermore, these composites exhibited high photoluminescence (PL) quantum yields of up to 60.63%, and their LPL was robust under ambient conditions, even in aqueous media. Their robust and superior LPL performance endows these composites with a strong competitive advantage in dynamic display systems, such as tags for time‐resolved data encryption and displays of the remaining time of takeaways. This study offers an approach to preparing CDs‐based LPL composites with tunable durations and may provide new insights for the development of rare‐earth‐free LPL materials.

HLCT-Type Acceptor Molecule-Based Exciplex System for Highly Efficient Solution-Processable OLEDs with Suppressed Efficiency Roll-Offs.

Exciplex systems are promising candidates for thermally activated delayed fluorescence (TADF) molecules because of the small energy difference between the lowest singlet and triplet excited states (ΔEST). However, realizing high-efficiency and low-external-quantum-efficiency (EQE) roll-off in solution-processed organic light-emitting diodes (OLEDs) using an exciplex system remains a formidable challenge. In this study, two (HLCT)-type isomers with a spiro skeleton, 2-tBuspoCz-TRZ and 10-tBuspoCz-TRZ, are designed and synthesized as acceptors of exciplexes, where tert-butylspirofluorene indole is regarded as a donor and the triazine unit as an acceptor. Green exciplex emissions are observed for the 2-tBuspoCz-TRZ:TAPC and 10-tBuspoCz-TRZ:TAPC exciplexes, indicating distinct TADF characteristics with a very small ΔEST of 35 ± 5 meV. By using the TADF exciplex system based on the HLCT acceptor as an emitter, solution-processable OLEDs achieve a maximum external quantum efficiency (EQEmax) of 20.8%. Furthermore, a high EQEmax > 25% with a very low-efficiency roll-off (≈3.5% at 1000cd m-2) is obtained for solution-processable phosphorescent devices using HLCT-based exciplexes as the host matrix of phosphors. This study paves the way for a novel strategy for designing acceptor exciplex molecules for effective TADF molecules and host matrices in solution-processable OLEDs.

Intramolecular exciplex featuring a bis-sp3 C-locked acceptor-donor-acceptor sandwich.

Intramolecular exciplex systems featuring thermally activated delayed fluorescence (TADF) have garnered significant attention in the realm of organic light-emitting diodes (OLEDs). Nonetheless, the occurrence of organic sandwich intramolecular exciplexes remains rare due to structural limitations and synthetic challenges. Herein, we present a novel rigid acceptor-donor-acceptor (A-D-A) sandwich complex, dSFQP, characterized by two sp3 C-locking moieties. This compound exhibits TADF characteristics facilitated by a multiple through-space charge-transfer process. X-ray crystallographic analysis confirms the distinctive sandwich configuration. The parallel spatial arrangement and minimized A-D-A configuration enhance electronic interactions, resulting in a high photoluminescence quantum yield, rapid reverse intersystem crossing rate, and sluggish nonradiative decay rate. OLEDs employing dSFQP as the dopant achieve a maximum external quantum efficiency (EQE) of 28.5% with a low efficiency roll-off of merely 2.8% at 1000 cd m-2. Even at a high brightness of 10 000 cd m-2, the EQE remains notably high at 17.5%. Our current results provide an effective way to further innovate the design of new organic charge-transfer complexes.

Primary photodegradation pathways of an exciplex-forming A–D molecular system

Photodegradation study on Py-DMA exciplex system identified demethylation and oxidative formylation as primary pathways, with oxygen included. The results, validated by various analyses, offer insights into extending optoelectronic device lifespan.

One-step synthesis of color-tunable carbon dots-based organic long persistent luminescence materials

The use of carbon dots (CDs) to construct binary donor–acceptor exciplex systems has been shown to be an effective method for obtaining organic long persistent luminescence (OLPL). However, such a system currently necessitates the prior synthesis of CDs before integration with the organic matrix, thus complicating the reproducibility and large-scale production of the materials. Furthermore, attaining color-tunable long persistent luminescence (LPL) is also a major challenge for the system. Here, we report a one-step synthesis strategy for color-tunable CDs-based OLPL material (named CDs@CA). CDs donors and CA acceptors in the material can be generated in a single step during the reaction. Under ambient conditions and even in aqueous media, the LPL color of CDs@CA can be changed by the excitation wavelength, enabling color tuning from blue to green emission. This is the first time for the color tuning to be achieved in CDs-based OLPL materials. Meanwhile, the strategy is applicable to a wide range of alternative carbon sources, which makes it possible to rapidly prepare a wide range of CDs-based OLPL materials with varying LPL performance by selecting different carbon sources. This work is greatly beneficial to the development and application of CDs-based OLPL systems.

Boosting Phosphorescence Efficiency of Microstructures by Forming a Triplet Exciplex System

AbstractLow‐dimensional crystal structures with long lifetime phosphorescence hold great potential in biological imaging, sensors, and micro/nanophotonics. However, the high‐efficient phosphorescence is still scarce on the micro/nanoscale due to the strong nonradiative transitions and quenching of long‐lifetime triplet state caused by the large specific surface area. Herein, a one doped microstructure with high quantum yield phosphorescence is reported by doping an electron‐acceptor (guest) into an electron‐donor (host). The formation of a triplet exciplex between the donor and acceptor enables the doped microcrystals to display highly efficient long‐lived emission with 63.1% quantum yield, while the phosphorescence emissions of microstructures from a neat donor and acceptor are negligible. The emission mechanism in relation to the exciplex is elaborated from temperature‐dependent luminescence behavior and theoretical calculation. This work provides an effective strategy for achieving efficient long‐lived emissions at the micro‐scale, which will accelerate the research and development of advanced miniaturized functional devices.

Forthcoming hyperfluorescence display technology: relevant factors to achieve high-performance stable organic light emitting diodes.

Over the decade, there have been developments in purely organic thermally activated delayed fluorescent (TADF) materials for organic light-emitting diodes (OLEDs). However, achieving narrow full width at half maximum (FWHM) and high external quantum efficiency (EQE) is crucial for real display industries. To overcome these hurdles, hyperfluorescence (HF) technology was proposed for next-generation OLEDs. In this technology, the TADF material was considered a sensitizing host, the so-called TADF sensitized host (TSH), for use of triplet excitons via the reverse intersystem crossing (RISC) pathway. Since most of the TADF materials show bipolar characteristics, electrically generated singlet and triplet exciton energies can be transported to the final fluorescent emitter (FE) through Förster resonance energy transfer (FRET) rather than Dexter energy transfer (DET). This mechanism is possible from the S1 state of the TSH to the S1 state of the final fluorescent dopant (FD) as a long-range energy transfer. Considering this, some reports are available based on hyperfluorescence OLEDs, but the detailed analysis for highly efficient and stable devices for commercialization was unclear. So herein, we reviewed the relevant factors based on recent advancements to build a highly efficient and stable hyperfluorescence system. The factors include an energy transfer mechanism based on spectral overlapping, TSH requirements, electroluminescence study based on exciplex and polarity system, shielding effect, DET suppression, and FD orientation. Furthermore, the outlook and future positives with new directions were discussed to build high-performance OLEDs.

Highly efficient non-doped organic light-emitting diodes based on long-range coupling and efficient energy transfer

An effective approach to realize high-efficiency exciplex-based organic light-emitting diodes (OLEDs) is proposed by employing long-range coupling between electron-hole pairs and efficient energy transfer. The space-separated holes and electrons form exciplexes through charge-transfer states, and devices with such structures can reduce exciton quenching in a certain extent compared to those based on interfacial exciplexes. Remarkable external quantum efficiency about 12.6% is achieved by introducing a yellow exciplex system in a donor/spacer/acceptor structure because of the suppression of exciton quenching and the efficient energy transfer. Such a unique strategy is beneficial to better utilizing the triplet excitons and thus achieving high performance exciplex-based OLEDs.

Thermally Activated Long Persistent Luminescence of Organic Inorganic Metal Halides

AbstractLong persistent luminescence (LPL) materials of SrAl2O4 doped with Eu2+ or Dy3+ can maintain emission over hours after ceasing the excitation but suffer from insolubility, high cost, and harsh preparation. Recently, organic LPL of guest‐host exciplex systems has been demonstrated via an intermediate charge‐separated state with flexible design but poor air‐stability. Here, we synthesized a nontoxic two‐dimensional organic–inorganic metal hybrid halides (OIMHs), called PBA2[ZnX4] with X=Br or Cl and PBA=4‐phenylbenzylamine. These materials exhibit stable LPL emission over minutes at room‐temperature, which is two orders of magnitude longer than those of previously reported OIMHs. The mechanism study shows that the LPL emission comes from thermally activated charge separation state rather than room‐temperature phosphorescence. Moreover, the LPL of PBA2[ZnX4] can be excited by low power sources, representing an effective strategy for developing low‐cost and high‐stability LPL systems.

Thermally Activated Long Persistent Luminescence of Organic Inorganic Metal Halides.

Long persistent luminescence (LPL) materials of SrAl2 O4 doped with Eu2+ or Dy3+ can maintain emission over hours after ceasing the excitation but suffer from insolubility, high cost, and harsh preparation. Recently, organic LPL of guest-host exciplex systems has been demonstrated via an intermediate charge-separated state with flexible design but poor air-stability. Here, we synthesized a nontoxic two-dimensional organic-inorganic metal hybrid halides (OIMHs), called PBA2 [ZnX4 ] with X=Br or Cl and PBA=4-phenylbenzylamine. These materials exhibit stable LPL emission over minutes at room-temperature, which is two orders of magnitude longer than those of previously reported OIMHs. The mechanism study shows that the LPL emission comes from thermally activated charge separation state rather than room-temperature phosphorescence. Moreover, the LPL of PBA2 [ZnX4 ] can be excited by low power sources, representing an effective strategy for developing low-cost and high-stability LPL systems.

Extending Anisotropy Dynamics of Light‐Emitting Dipoles as Necessary Condition Toward Developing Highly‐Efficient OLEDs

AbstractDesigning in‐plane‐oriented light‐emitting dipoles is known as a critical method to develop high‐efficiency organic light‐emitting diodes (OLEDs) by enhancing light extraction. However, in‐plane‐oriented light‐emitting dipoles must demonstrate sufficient polarization memory extended into light emission lifetime window, generating extended anisotropy dynamics shown as the necessary condition to increase light extraction toward developing high‐efficiency OLEDs. This paper reports experimental studies on anisotropy dynamics of light‐emitting dipoles in both time and energy domains by using time‐resolved and steady‐state photoluminescence anisotropy measurements based on the in‐plane oriented exciplex‐heterostructured [BCzPh:CN‐T2T] host dispersed with phosphorescent molecules. It is found that, when host–guest Coulomb scattering is suppressed by parallel placing of the in‐plane‐configured phosphorescent Ir(ppy)2(acac) molecules into the in‐plane‐oriented exciplex‐heterostructured [BCzPh:CN‐T2T] host, the anisotropy dynamics of light‐emitting dipoles can be extended into microseconds time window comparable with its phosphorescence lifetime, satisfying the necessary condition in time domain to increase light out‐coupling efficiency toward developing high external quantum efficiencies (EQEs) in Ir(ppy)2(acac):exciplex system. More importantly, by suppressing host–guest Coulomb scattering, the high‐energy transition dipoles can still maintain extended anisotropy dynamics in the energy domain in Ir(ppy)2(acac):exciplex system while hot electrons are relaxing toward lowest unoccupied molecular orbital (LUMO). Consequently, the extended anisotropy dynamics of light‐emitting dipoles demonstrate a high EQE of 34.01% in the Ir(ppy)2(acac):exciplex OLED.

Highly efficient and low efficiency roll-off single-emissive-layer white organic light emitting devices by quaternary exciplex

The challenge of achieving both highly efficient and low-efficiency roll-off single-emissive-layer (SEL) white organic light-emitting diodes (WOLEDs) are still the balance charge transfer and precise manipulation of singlet and triplet. In this context, a novel quaternary exciplex system is fabricated, with blue thermally activated delayed fluorescence (TADF) materials DMAC-DPS, TPXZPO, and traditional fluorescent material MCP were selected as electron-donating materials, and PO-T2T was selected as the electron-accepting material. Three different exciplexes with dissimilar emission wavelengths are formed in the SEL. The quaternary exciplex can be regarded as MCP:PO-T2T exciplex host dopant DMAC-DPS:PO-T2T exciplex and TPXZPO:PO-T2T exciplex. The effective exciton confinement and multiple reverse intersystem crossing (RISC) channels are achieved by doping high triplet TADF electron-donating material, which reduces energy loss and promotes the utilization of excitons. In comparison to the binary and ternary exciplex devices, the quaternary exciplex device exhibited the highest external quantum efficiency of 16.68% due to higher rate constants of RISC (5.96 × 106 s−1) and photoluminescence quantum yield (PLQY). Especially, the internal exciplex to exciplex stepped energy transfer channels ensure slightly efficient roll-off and concentration-independent color shifts in the EL spectra. The external quantum efficiency remained at 16.60% and 15.74% at a luminance of 1000 cd/m2 and 5000 cd/m2, respectively. Therefore, these results indicate that quaternary exciplex has a promising future in the applications of high-performance SEL all-fluorescence WOLEDs.

Chiral Exciplex Acceptor Enables Circularly Polarized Electroluminescence with High Dissymmetry Factor Close to 10−2

AbstractChirality is a magical property that exists in nature universally, ranging from the spin of microscopic electrons to the galaxies. Furthermore, circularly polarized organic electroluminescent materials have promising prospects in improving the brightness of displays, but there are still some problems. At present, most of chiral thermally activated delayed fluorescence small molecules show relatively low circularly polarized luminescence (CPL) dissymmetry factors between 10−4 and 10−3 because of the electric dipole‐allowed but magnetic dipole‐forbidden transition of the singlet state. In this work, a promising strategy is reported for constructing chiral exciplex systems with a chiral electron acceptor and a regular achiral donor to achieve high CPL dissymmetry factors in both photoluminescence and electroluminescence. From theoretical calculation results, the exciplex systems possess a large separation of the highest occupied molecular orbital and the lowest unoccupied molecular orbital in donor and acceptor molecules, respectively, which results in comparable electric and magnetic transition dipole moments. The resultant CPL organic light‐emitting diodes based on the exciplex systems as emitters show low turn‐on voltage of 2.4 V, high external quantum efficiencies of 12.7% and 9.3%, and opposite circularly polarized electroluminescence with high dissymmetry factors (gEL) of +7.25 × 10−3 and −9.89 × 10−3.